Direct current negative feedback amplifier



Jan. 2, 1951 FIEL B. L. WELLER DIRECT CURRENT NEGATIVE FEEDBACK AMPLIFIER Filed Feb. 27. 1945 'FUE 4- y," @ma zzfaye @ma wavy@ Patented Jan. 2, 19u51 DIRECT CURRENT NEGATIVE FEEDBACK AMPLIFIER Barton L. Weller, Richland, Wash., assigner to the United States of America as represented by the United States Atomic Energy Commission Application February 27, 1945, Serial No. 580,044

2 Claims.

The present invention relates to an electromv eter circuit, more particularly to a degenerative or inverse feedback, direct current amplifier circuit including an electrometer tube, which circuit is particularly suitable for measuring small currents, such as those developed in a radiation responsive ion chamber.

In the past, direct current electrometer circuits involving an inverse or negative feedback feature have been used for measuring small cur- 2 Referring more particularly to Fig. 1, numeral I denotes an ion chamber including a cylindrical electrode or anode 2, and a centrally disposed collecting electrode or cathode 3. Chamber l is lled with argon or other suitable gas that readily becomes ionized when subjected to radiations, such as gamma radiations, resulting.r in the develrents because of the stability imparted to the Aj circuit, rendering the performance substantially independent of changes in tube characteristics, slow variations in supply voltages, etc., thereby effectively extending the range of indicating and recording instruments although with sacrice in the amplication factor of the systern. However, transient variations occurring in different portions of such circuits, such as those caused by sudden changes in tube characteristics, or the dropping off of pieces of oxide coating from the cathode, and rapid changes in supply voltages or in the quantity being measured, have given rise to considerable difficulty in obtaining. steady and accurate measurements of small ion currents being developed. Many believe that such variations are principally due to the short time constant of the circuit, changes of which are thought to be difcult to detect by ordinary instruments. I found that interpretation to be incorrect because removal of a large resistor in the feedback circuit causes disappearance of this transient instability.

An object of my invention is to provide a degenerative or inverse feedback electrometer circuit that is substantially devoid of transient instability.

A more specific object of my invention is to provide a direct current amplifier, including an electro neter tube, incorporating an inverse feedback circuit that considerably increases the stability when used to measure small currents, such as io-n currents developed in a radiation responsive chamber.

Other objects and advantages will become apparent from the following specification when read in view of the drawing wherein:

l is a schematic circuit diagram of a direct current electrometer circuit incorporating my invention and involving inverse feedback, that is coupled to a radiation responsive ion chamber; and,

1Figs. 2, 3, and 4 are graphs of voltage vari-1 ations of the electrometer tube grid input plotted against time for three different values of capacity of condenser ll shown in Fig. 1;

Some

opment of ions. The ions being positive, are collected by collecting electrode 3 which is maintained by a potential source marked DC at a negative potential with respect to the anode 2, thereby forming an ion current that is subsequently amplified and measured. A guard ring i is interposed by insulators (not shown) between the collecting electrode 3 and high potential electrode or anode 2 so as to act as an electrostatic shield to protect the collecting electrode from disturbing eiiects caused by the high potential diiierence between electrode 2 and the guard ring 4 and against the possibility of polarization of the collecting electrode insulator. Polarization is the separation of positive and negative charges on such insulator but not sufciently in extent as to be pulled out of their original atoms or molecules. Guard ring t, therefore, protects against either surface charges or virtual charges due to polarization of the insulator which would otherwise affect the potential of the collecting electrode 3 and introduce errors in measurement.

`An eleotrometer direct coupled ampliiier circuit is provided to amplify the ion currents develcped and includes an odd number of stages or electric discharge tubes, T1, T2, and T3, each stage providing a change of phase between the input and output voltages as well as providing amplification. Tube T1 is an electrometer tube having an input grid 5 directly connected to collecting electrode 3 of the ion chamber l. Tubes T1, T2, and T3, preferably of the electronic triode type as shown, are directly coupled as indicated and are provided with suitable anode resistors 6, l, and 8 as well as a plurality of bias batteries S, it, and H, the latr having values of 4 to 10 volts, for example. A battery i2 having a voltage of around 90 volts provides the plate voltage across the output of tube T3. A microammeter i3 is provided in the output circuit for measuring output currents which are indicative of the radiation intensities falling on ion chamber I. A tap on battery l2 is adjusted so that the plate voltage of tube T3 will be neutralized and cause no current flow through meter i3 for zero signal from chamber l.

A high impedance or resistor it having a value, for example, of 1011 ohms, is connected directly to the collecting electrode 3 and input grid 5 and between these electrodes and the plate of tube T3 and battery 9 through resistors l and I5 respectively. Thus the output voltage of tube T3 is applied across resistors If and I5 which effectively form a voltage divider with an intermediate tap to the resistor ib. Resistors I4 and I5 may be of the order of 103 ohms. Thus it will be seen that by suitably adjusting the ratios of resistors I4 and l5 a denite fraction of the output voltage may be fed through resistor I6 to the input grid 5. The voltage of input grid 5 varies with the voltage across resistor IS which in turn varies with changes in ion current flow through chamber i. The inverse feedback principle dictates that the voltage of point I8, that is the tap point of resistors I4 and I5, changes in a direction t oppose a change in voltage across resistor I6 created by ionization currents from ion chamber I. This action results from the condition that the input grid changes in potential only times the change in voltage across resistor IG, where G is the overall gain of the entire ampliiier circuit and K is the proportion of the output voltage fed into the input. As G is usually 1000 or more, the change in the potential of grid is relatively small. In order to reduce the long time constant inherent with such high impedance i6, the guard ring Q is returned to a constant potential source at substantially the same potential as that of the input grid, such as may be eiected by connecting the guard ring to a grounded negative terminal of bias battery 9. The guard ring Ii is thus slightly negative in potential with respect to the cathode of tube T1. Since the inverse feedback operation allows grid 5 to vary only a small amount, such as of the order of less than 1/1090 volt the change of potential across the grid-to-guard capacity is much smaller than the change across the high resistor I6. The'charging time is, therefore, reduced and the time constant of the circuit is shortened and thereby improved. This desirable arrangenient, however, detracts from satisfactory feedback operation because the grid-to-guard capacity and the high impedance resistor Iii do not allow the feedback to act instantaneously. Thus the feedback cannot operate for fast variations, hence the full gain of the amplier carries these variations to the output meter I3.

One method of preventing this condition is to by-pass the high resistor I5 with some capacity. rThis arrangement, however, is undesirable because the time constant is increased in the same proportion as the impedance and stability is reduc-ed. Another method is to supply some feedback through the grid-to-guard capacity by supplying some feedback voltage to the guard ring This method is an improvement but does not completely solve the problem unless the guard ring is returned to the same point or voltage value as the high resistor I5. By this method many of the advantages of the low time constant are lost because the grid-to-guard capacity has to be charged to the full input potential.

In accordance with my invention, as shown in Fig. 1. voltage transient eifects are eliminated by `providing a parallel impedance circuit in the inverse feedback network, which circuit cornprises a pair of parallel paths, one of which includes a small variable condenser I'! having a capacity, for example, between 1 and 10 mmf., and the other parallel path includes the high value resistor I6. Without such condenser Il, the high impedance of the resistor I6 and the inherent input capacity including the grid-to-guard capacity would be eective to prevent the feedback from responding to rapid voltage variations in the circuit or to higher frequency components of such variations, particularly those above 5 cycles per second.

Fig. 2, curve A shows the relatively slow change in the value of grid voltage of tube T1, exclusive of the effects of condenser i1, due to the relatively high tirne constant provided by the high value of resistor I 6 and the grid-to-guard capacity. It will be noted that condenser I? by virtue of its direct connection to the plate of the output tube Ts will receive feedback voltage of substantially greater amplitude than the reduced feedback voltage fed through the resistor It, perhaps over four times as large. Hence, as indicated by curve B of Fig. 2, which shows variations in value of grid voltage of tube T1 due solely to the condenser Il, the grid voltage instantaneously rises to a relatively high value and will afterwards gradually decrease due to the subsequent discharge of condenser Il through resistors IE and I4. By properly selecting the value of the capacity of condenser il, generally between 1 and l0 mmf., it is possible to get an optimum curve C shown in Fig. 2, which is the resultant of curves A and B and represents the resultant grid voltage change from the circuit as illustrated in Fig. 1. Such optimum curve C is one that substantially instantaneously as- .surnes a predetermined value and maintains that value during the period that condenser Il' discharges and the grid-to-guard capacity is charged. The value of the condenser Il, therefore, is critical because if the capacity is too large, that is, if it is substantially greater than 10 mmf., the peak of curve B of Fig. 2 will be greater, and the resultant curve will assume the shape shown in Fig. 3, which shows a disturbing transient voltage due to the under-damped condition existing. On the other hand if the value of condenser Il is too small, the peak of curve B of Fig. 2 will likewise be small resulting in a grid voltage, such as shown in Fig. 4, which is illustrative of an over-damped condition resulting in sluggish and undesirable operation. The improvement in the time constant by using condenser i? is about 1000 fold.

It will be seen by virtue of the higher amplitude 'or swing of feedback voltage applied to condenser I1, that this condenser can be relatively small and still allow the input grid 5 to be driven in spite of the grid-to-guard capacity and to respond to rapid voltage changes or socalled rst derivative changes of voltage, particularly those above 5 cycles per second. In this manner rapid change occurring in any part of the circuit Will be rapidly reflected and cornpensed for by the feedback circuit. Thus it will be seen that rapid voltage fluctuations particularly above 5 cycles per second will be bypassed to the grid ,5 by condenser Il', whereas slow iiuctuations inv voltage, mainly below about 5 cycles per second will be transferred to grid 5 through resistor I5.

nSince the condenser I'I is of small capacity, it can be included without introducing appreciable additional leakage paths in the electrometer circuit. Condenser I1 being small can be asador? mounted on parts that already exist in the circuit, such as the lead-in to input grid 5, and may be in the form of an air condenser or in the form of a cable of vvariable length.

Best operation is obtained insofar as stability is concerned when either no external current or a Very small external current from the ion chamber I flows through the high resistor I6. High values of ion current flow through the high value resistor I6 have a tendency to give rise to a small degree of voltage transients.

Three stages of amplication have been illustrated in Fig. 1. It should be noted than any number of odd stages may be used instead, and further that the feedback voltage may be fed from any of the latter odd stages, not necessarily the last stage. Furthermore, while the electrometer feedback has been described in connection with a radiation measuring device, 1t

should be noted that such circuit is likewise adaptable to any other circuit where small input currents are to be measured or detected. Other alternate arrangements likewise may be readily suggested to those skilled in the art after having had the benefit of the teachings of my invention. For this reason, the invention should not be limited except insofar as set forth in the following claims.

I claim:

1. In a high-impedance inverse-feedback direct current electronic amplifier, an input grid resistor of a value of the order of 1011 ohms, a source of negative grid bias connected in series with the grid resistor, a second resistor connected between the grid resistor and the source of negative grid bias, an output circuit including a junction having a potential varying oppositely to variations in the potential of the input grid, a third resistor connecting 'said junction to the junction of the second resistor and the grid resistor, and a condenser having a capacity between 1 and 10 mmf. connected directly between said junction in the output circuit and the grid of the input tube, said capacity being critically damped by the input grid resistor,

whereby the voltage applied to the input grid consists of signal voltage and negative feedback voltage and the sum of these voltages rapidly assumes a steady state upon a change in input signal voltage.

2. A high impedance inverse-feedback direct current electronic amplifier comprising, in combination, an odd number of cascaded directcoupled amplier stages, a grid resistor having one terminal connected to the grid of the input tubehaving a value of the order of 1011 ohms, a source of negative grid bias connected in series with the grid resistor, a second resistor connected between the grid resistor and the source of negative grid bias, a third resistor,` connecting the junction of the grid resistor and the second resistor directly to the plate of the last stage, and a condenser having a capacity between 1 and 10 mmf. connected directly between the plate of the last stage and the input grid, said capacity being critically damped by the input grid resistor, whereby the voltage applied to the input grid consists of signal voltage and negative feedback voltage and the sum of these kvoltages rapidly assumes a steady state upon a change in input signal voltage.

BARTON L. WELLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,915,440 Nyquist June 27, 1933 2,033,963 Ware Mar. 17, 1936 2,162,412 Victoreen June 13, 1939 2,173,426 Scott Sept. 19, 1939 2,202,522 Gloess May 28, 1940 2,224,699 Rust Dec. 10, 1940 2,245,671 Holst June 17, 1941 2,297,543 Eberhardt Sept. 29, 1942 2,401,779 Swartzel, Jr., June 11, 1946 2,431,335 Langmuir Nov. 25, 1947 

